The present invention relates to motors and, more particularly, to a magnet assembly and to a linear motor incorporating the magnet assembly.
Linear motors are used in various types of systems, such as for positioning and moving applications, including machining and gantry type systems. The high performance systems often require moving elements subjected to high acceleration levels. In order to achieve such high acceleration, the linear motor must exert large forces upon the elements to be moved.
There are various configurations of linear motors, including flat motors, U-channel motors and tubular shaped motors. Different types of linear motors are also available, including brush, AC brushless, stepper, and induction motors. Common to most linear motors is a moving assembly, usually called a forcer or stage, which moves relative to a stationary platen (or path) according to magnetic fields generated by application of current through one or more associated windings. The windings can be on the forcer or at the platen depending on the type of motor. For example, in a permanent magnet linear motor, a series of armature windings can be mounted within a forcer that is movable relative a stationary path. The path can include an array of permanent magnets configured to interact with the coils in the stage when energized with an excitation current.
Alternatively, in another type of conventional linear motor, permanent magnets can be part of a moveable stage with the coils situated in the platen. Usually, the permanent magnets are attached to a back iron plate above the coils, which are oriented along a path of travel. The magnets usually are rectangular in shape. The magnets are arranged along the back iron so that adjacent pairs of magnets have opposite magnetic pole orientations. The magnets can be oriented generally normal to the direction of travel or inclined at a slight angle from normal to an axis of the direction of travel for the linear motor. The inclined angle creates a flux distribution along the axis of movement which is generally sinusoidal in nature. Such a resulting distribution due to the optimized motor geometry tends to reduce cogging during operation of the linear motor, which would otherwise occur if the magnets were aligned, normal to the axis of movement.
Although an inclined angle of the magnets can reduce some cogging, it presents a disadvantage in that a larger area typically must be covered by the rectangular magnets in order to sufficiently cover and interact with the coils of the armature. When the magnets are implemented with a larger area so as to reduce cogging effects, a larger footprint for the back iron also is required. This tends to increase the overall weight and size of the stage. Such increases in size and weight can present additional obstacles, such as in applications were there are size constraints and low mass is desirable. For example, as the mass of the stage increases, the available acceleration experiences a corresponding reduction, and the ability to stop the motor accurately also reduces because of the increased power dissipation needed to stop the motor.
As the use of linear motors in manufacturing equipment continues to increase, nominal increases in the speed of operation translate into significant savings in the cost of production. Accordingly, it is desirable to provide a magnet assembly that can be part of a high performance linear motor.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the present invention provides a magnet assembly that can be employed as part of a linear motor stage to form a high performance linear motor. The magnet assembly includes a plurality of magnets operatively associated with magnetically conductive plate, commonly known as a back iron. The magnets extend from a common side of the back iron. The back iron is dimensioned and configured to substantially conform to magnetic flux that travels through the back iron when the magnet assembly is exposed to a magnetic field, such as from windings of a motor path. In one particular aspect of the present invention, a cross-sectional dimension of the back iron varies between opposed ends of the back iron as a function of the position and/or orientation of the magnets. For example, a thickness of the back iron is greater at locations between adjacent pairs of the magnets than at locations generally centered with the respective magnets. As a result of such back iron geometry, force output to moving mass ratio of a motor incorporating the magnet assembly is improved over conventional configurations of magnet assemblies. Also, the back iron geometry reduces leakage flux.
Another aspect of the present invention provides a linear motor system that includes a path having a plurality of windings, which can be energized to produce desired magnetic fields. The linear motor system also includes a magnet assembly, such as described above. The linear motor system achieves high performance because the magnet assembly has a reduced mass, which substantially conforms to magnet flux lines that travel through the magnet assembly during energization of path windings. The mass further can be reduced by employing generally elongated octagonal magnets, such as by removing corner portions from rectangular magnets.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention are described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.